1. Technical Field
The present disclosure relates to a steering reaction force control apparatus for a vehicle including an electric power steering device and a driving support device.
2. Description of the Related Art
A steering reaction force control apparatus for a vehicle, e.g., a motor vehicle, is configured to apply a torque to a steering device through an electric power steering device, thereby reducing a steering load imposed on a driver, and improving steering feeling. For example, as described in Japanese Patent Application Laid-open No. 2007-76582, in a steering reaction force control apparatus, a target steering reaction force torque is calculated, and an electric power steering device is controlled through PID feedback so that an actual steering reaction torque reaches the target steering reaction force torque. The target steering reaction force torque includes a target spring torque for generating a force of returning a steering wheel to a neutral position, and further includes a target attenuation torque for generating a steering resistance force proportional to a steering angular velocity to increase stability of a steering system.
Incidentally, as disclosed in Japanese Patent Application Laid-open No. 2013-193490, in a vehicle including a driving support device configured to carry out automatic driving or the like, a control device for an electric power steering device is configured to operate in two operation modes including a manual steering mode and an automatic steering mode, and the operation mode is chosen by a passenger of the vehicle operating a switch.
In any one of the manual steering mode and the automatic steering mode, the control device for the steering reaction force control apparatus is configured to calculate a target steering reaction force torque, and control the electric power steering device through the PID feedback so that an actual steering reaction torque reaches the target steering reaction force torque.
In the automatic steering mode, the electric power steering device also functions as an actuator configured to generate a steering torque for automatically steering the steered wheels. The driving support device is configured to calculate a target steered angle of the steered wheels for causing the vehicle, for example, to travel along a traveling lane, and the control device is configured to calculate a target steering reaction force torque based on a deviation between the target steered angle and an actual steered angle of the steered wheels. Further, the control device is configured to control the electric power steering device through the PID feedback so that the steering reaction torque reaches the target steering reaction force torque, and as a result, the steering reaction force is controlled, and the steered angle of the steered wheels is also controlled so as to reach the target steered angle.
Specifically, the target steering spring torque is calculated based on the steering angle corrected by the target steering angle corresponding to the target steered angle, and the target steering attenuation torque is calculated based on a time derivative of the steering angle or a time derivative of the corrected steering angle. As a ratio of the target steering spring torque to the steering angle or the corrected steering angle increases, the steering reaction force felt by the driver increases, and the steering assist torque thus consequently decreases.
In general, a gain when the electric power steering device is controlled through the PID feedback is constant in the steering reaction force control apparatus. Therefore, when the gain of the feedback control is set to a low value by reducing a resistance force due to a steering attenuation torque in order to improve a property of the steered wheels to follow the target steered angle in the automatic steering mode, the steering stability decreases when the driver conducts override steering. Conversely, when the gain of the feedback control is set to a high value in order to secure the operability of the override steering when the driver conducts override steering, a resistance force due to a steering attenuation torque increases, which deteriorates the property of the steered wheels to follow the target steered angle in the automatic steering mode.
Notably, when the gain of the feedback control is set to a high value by making a steering spring torque be large in magnitude in order to improve a property of the steered wheels to follow the target steered angle in the automatic steering mode, the steering reaction force felt by the driver increases when he or she steers in a direction where an actual steered angle of the steered wheels is changed away from the target steered angle, and an operability of override steering, namely, an operability by which the driver performs steering while overriding the automatic steering deteriorates. Conversely, when the gain of the feedback control is set to a low value by making a steering spring torque be small in magnitude in order to secure the operability of the override steering, the property of the steered wheels to follow the target steered angle in the automatic steering mode deteriorates.